Over the years there has been much disagreement on the exact evolutionary relationships of the corvid family and their relatives. What eventually seemed clear was that corvids are derived from Australasian ancestors and from there spread throughout the world. Other lineages derived from these ancestors evolved into ecologically diverse, but often Australasian groups. Sibley and Ahlquist united the corvids with other taxa in the Corvida. The presumed corvid relatives included currawongs, birds of paradise, whipbirds, quail-thrushes, whistlers, monarch flycatchers and drongos, shrikes, vireos and vangas,[3] but current research favors the theory that this grouping is partly artificial. The corvids constitute the core group of the Corvoidea, together with their closest relatives (the birds of paradise, Australian mud-nesters and shrikes). They are also the core group of the Corvida, which includes the related groups, such as orioles and vireos.[4]

Fossil record

The earliest corvid fossils date to the mid-Miocene, about 17 million years ago; Miocorvus and Miopica may be ancestral to crows and some of the magpie lineage, respectively, or similar to the living forms due to convergent evolution. The known prehistoric corvid genera appear to be mainly of the New World and Old World jay and Holarctic magpie lineages:

In addition, there are numerous fossil species of extant genera since the Mio–Pliocene, mainly European Corvus.[9]

Biology

Morphology

Corvids are medium to large in size, with strong feet and bills, rictal bristles and a single moult each year (most passerines moult twice). Corvids are found worldwide except for the tip of South America and the polar ice caps.[10] The majority of the species are found in tropical South and Central America, southern Asia and Eurasia, with fewer than 10 species each in Africa, Australasia and North America. The genus Corvus has re-entered Australia in relatively recent geological prehistory, with five species and one subspecies there.

Corvids are large to very large passerines with a robust build, strong legs and all species except the Pinyon Jay have nostrils covered by bristle-like feathers.[11] Many corvids of temperate zones have mainly black or blue coloured plumage; however, some are pied black and white, some have a blue-purple iridescence and many tropical species are brightly coloured. The sexes are very similar in color and size. Corvids have strong, stout bills and large wingspans. The family includes the largest members of the passerine order.

The smallest corvid is the Dwarf Jay (Aphelocoma nana), at 40 g (1.4 oz) and 21.5 cm (8.5 inches). The largest corvids are the Common Raven (Corvus corax) and the Thick-billed Raven (Corvus crassirostris), both of which regularly exceed 1400 grams (3 lbs) and 65 cm (26 inches).

Species can be identified based on size, shape, and geography; however, some, especially the Australian crows, are best identified by their raucous calls.[3]

Ecology

Corvids occur in most climatic zones. Most are sedentary and do not migrate significantly. However, during a shortage of food, eruptive migration can occur.[3] When species are migratory, they will form large flocks in the fall (around August in the northern hemisphere) and travel south.[12]

One reason for the success of crows, compared to ravens, is their ability to overlap breeding territory. During breeding season, crows were shown to overlap breeding territory six times as much as ravens. This invasion of breeding ranges allowed a related increase in local population density.[13]

Food and feeding

The natural diet of many corvid species is omnivorous, consisting of invertebrates, nestlings, small mammals, berries, fruits, seeds, and carrion. However, some corvids, especially the crows, have adapted well to human conditions and have come to rely on anthropogenic foods. In a US study of American Crows, Common Ravens and Steller's Jays around campgrounds and human settlements, the crows appeared to have the most diverse diet of all, taking anthropogenic foods such as bread, spaghetti, fried potatoes, dog food, sandwiches, and livestock feed. The increase in available anthropogenic food sources is contributing to population increase in some corvid species.[13]

Some corvids are predators of other birds. During the wintering months, corvids typically form foraging flocks.[3] However, some crows also eat many agricultural pests including cutworms, wireworms, grasshoppers, and harmful weeds[12] Some corvids will eat carrion, and since they lack a specialized beak for tearing into flesh, they must wait until animals are opened, whether by other predators or as roadkill.

Since crows do not seem to mind human development, it was suggested that the crow population increase would cause increased rates of nest predation. However, Steller's Jays, which are successful independently of human development, are more efficient in plundering small birds' nests than American Crows and Common Ravens. Therefore, the human relationship with crows and ravens did not significantly increase nest predation, compared to other factors such as habitat destruction.[13]

Reproduction

Many species of corvid are territorial, protecting territories throughout the year or simply during the breeding season. In some cases territories may only be guarded during the day, with the pair joining off-territory roosts at night. Some corvids are well known communal roosters. Some groups of roosting corvids can be very large, with a roost of 65,000 Rooks counted in Scotland.[14] Some, including the Rook and the Jackdaw, are also communal nesters.

The partner bond in corvids is extremely strong and even lifelong in some species. This monogamous lifestyle, however, can still contain extra-pair copulations.[15] Males and females build large nests together in trees or on ledges. The male will also feed the female during incubation.[16] The nests are constructed of a mass of bulky twigs lined with grass and bark. Corvids can lay between 3 and 10 eggs, typically ranging between 4 and 7. The eggs are usually greenish in colour with brown blotches. Once hatched, the young remain in the nests for up to 6–10 weeks depending on the species. Corvids provide biparental care.

Social life

Some corvids have strong organization and community groups. Jackdaws, for example, have a strong social hierarchy, and are facultatively colonial during breeding.[17] Providing mutual aid has also been recorded within many of the corvid species.

Young corvids have been known to play and take part in elaborate social games. Documented group games follow a "king of the mountain"- and "follow the leader"-type pattern. Other play involves the manipulation, passing, and balancing of sticks. Corvids also take part in other activities, such as sliding down smooth surfaces. These games are understood to play a large role in the adaptive and survival ability of the birds.[19]

Mate selection is quite complex and accompanied with much social play in the Corvidae. Youngsters of social corvid species undergo a series of tests, including aerobatic feats, before being accepted as a mate by the opposite sex.[12]

Some corvids can be aggressive. Blue Jays, for example, are well known to attack anything that threatens their nest. Crows have been known to attack dogs, cats, ravens, and birds of prey. Most of the time these assaults take place as a distraction long enough to allow an opportunity for stealing food.[12]

Intelligence

They are considered the most intelligent of the birds, and among the most intelligent of all animals[20][21] having demonstrated self-awareness in mirror tests (European Magpies) and tool making ability (Crows, Rooks[22])—skills until recently regarded as solely the province of humans and a few other higher mammals. Their total brain to body ratio is equal to that of great apes and cetaceans, and only slightly lower than in humans.[23]

The brain-to-body weight ratios of corvid brains are among the largest in birds, equal to that of great apes and cetaceans, and only slightly lower than a human.[23] Their intelligence is boosted by the long growing period of the young. By remaining with the parents, the young have more opportunities to learn necessary skills. Since most corvids are cooperative brooders, their young can learn from different members of the group.[10]

When compared to dogs and cats in an experiment testing the ability to seek out food according to three-dimensional clues, corvids out-performed the mammals.[24] A metaanalysis testing how often birds invented new ways to acquire food in the wild found corvids the most innovative birds.[25] A 2004 review suggests that their cognitive abilities are on par with those of great apes.[26] Despite structural differences, the brains of corvids and great apes both evolved the ability to make geometrical measurements.

Corvid ingenuity is represented through their feeding skills, memorization abilities, use of tools, and group behaviour. Living in large social groups has long been connected with high cognitive ability. To live in a large group, a member must be able to recognize individuals and track the social position and foraging of other members over time. Members must also be able to distinguish between sex, age, reproductive status, and dominance, and to update this information constantly. It might be that social complexity corresponds to their high cognition.[27]

There are also specific examples of corvid cleverness. One Carrion Crow was documented to crack nuts by placing them on a crosswalk, letting the passing cars crack the shell, waiting for the light to turn red, and then safely retrieving the contents.[29] A group of crows in England took turns lifting garbage bin lids while their companions collected food.[citation needed]

Members of the corvid family have been known to watch other birds, remember where they hide their food, then return once the owner leaves.[citation needed] Corvids also move their food around between hiding places to avoid thievery, but only if they have previously been thieves themselves i.e., they remember previous relevant social contexts, use their own experience of having been a thief to predict the behavior of a pilferer, and can determine the safest course to protect their caches from being pilfered. Studies to assess similar cognitive abilities in apes have been inconclusive.[30]

The ability to hide food requires highly accurate spatial memories. Corvids have been recorded to recall their food's hiding place up to nine months later. It is suggested that vertical landmarks (like trees) are used to remember locations. There has also been evidence that Western Scrub-Jays, which store perishable foods, not only remember where they stored their food, but for how long. This has been compared to episodic memory, previously thought unique to humans.[10]

New Caledonian Crows (Corvus moneduloides) are notable for their highly developed tool fabrication. They make angling tools of twigs and leaves trimmed into hooks, then use the hooks to pull insect larvae from tree holes. Tools are engineered according to task and apparently also to learned preference. Recent studies revealed abilities to solve complicated problems, which suggests high level of innovation of a complex nature.[31] Other corvids that have been observed using tools include the American Crow, Blue Jay and Green Jay. Diversity in tool design among corvids suggests cultural variation. Again, great apes are the only other animals known to use tools in such a fashion.[10]

Clark's Nutcrackers and Jackdaws were compared in a 2002 study based on geometric rule learning. The corvids, along with a domestic pigeon, had to locate a target between two landmarks, while distances and landmarks were altered. The nutcrackers were more accurate in their searches than the jackdaws and pigeons.[32]

The scarecrow is an archetypal scare tactic in the agricultural business. However, due to corvids' quick wit, scarecrows are soon ignored and used as perches. Despite farmers' efforts to rid themselves of corvid pests, their attempts have only expanded corvid territories and strengthened their numbers.[12]

Current systematics places corvids, based on physical characteristics other than their brains (the most developed of birds), in the lower middle of the passerinesTemplate:Vague, contrary to earlier teleological classifications as "highest" songbirds due to their intelligence.[4] As per one observer:

During the 19th century there arose the belief that these were the 'most advanced' birds, based upon the belief that Darwinian evolution brings 'progress'. In such a classification the 'most intelligent' of birds were listed last reflecting their position 'atop the pyramid'. Modern biologists reject the concept of hierarchical 'progress' in evolution [...].

Disease

Corvids are reservoirs (carriers) for the West Nile Virus in the USA. They are infected by mosquitoes (the vectors), primarily of the Culex species. Crows and Ravens are quickly killed by this disease, so their deaths are an early-warning system when West Nile Virus arrives in an area (as are horse and other bird species deaths). One of the first signs that West Nile Virus first arrived in the USA in 1999 was the death of crows in New York.[33][34]

Relationship with humans

Several different corvids, particularly ravens, have occasionally served as pets, although they are not able to speak as readily as parrots and do not like being caged.[citation needed]

Role in myth and culture

Folklore often represents corvids as clever, and even mystical, animals. Some Native Americans, such as the Haida, believed that a raven created the earth and despite being a trickster spirit, ravens were popular on totems, credited with creating man, and responsible for placing the Sun in the sky.[citation needed]

The 6th century BC Greek scribe Aesop featured corvids as intelligent antagonists in many fables. Later, in western literature, popularized by American poet Edgar Allan Poe's work "The Raven", the Common Raven becomes a symbol of the main character's descent into madness.

Status and conservation

Unlike many other bird families, corvid fitness and reproduction, especially with many crows, has increased due to human development. The survival and reproductive success of certain crows and ravens is assisted by their close relationship with humans.[13]

Human development provides additional resources by clearing land, creating shrublands rich in berries and insects. When the cleared land naturally replenishes, jays and crows use the young dense trees for nesting sites. Ravens typically use larger trees in denser forests.[13]

The American Crow population of the United States has grown over the years. It is possible that the American Crow, due to humans increasing suitable habitat, will drive out Northwestern and Fish Crows.[39]